In a typical gas sensor, an infrared source is arranged to emit radiation, which passes through a gas to be sensed. Infrared radiation is absorbed by the gas and that remaining is subsequently detected by an infrared detector, such as a photodiode, thermopile or pyroelectric detector. A comparison is made between the source intensity and the intensity of radiation detected following passage through the gas to give the concentration of a target gas. The concentration is related to the intensity by the following equation:
  I  =      I    ·          ⅇ                        -          ε                ⁢                                  ⁢        c        ⁢                                  ⁢        l            
where I is the intensity of radiation detected by the detector, Io is the intensity of radiation emitted at the source, ε is effectively a constant which is dependent on the particular gas being monitored, c is the gas concentration and 1 is the distance travelled by the radiation through the gas.
We have appreciated problems with known detector arrangements. In particular, attempts have been made to fit multiple detectors into a single housing. Such an arrangement is shown in U.S. 2002/0063216, for example. However, such known systems suffer from inaccurate detection due to the physical limitations of the housing arrangement. We have appreciated the need, therefore, for an improved gas sensor containing multiple detectors within a housing.